This invention relates generally to semiconductor devices, and more particularly to equipment used in the assembly of semiconductor devices.
In the fabrication of most semiconductor devices, a chip or die is mechanically and electrically connected to the leads of a substrate prior to sealing the device in a protective package. Electrical contact is made between the conductive pads on the active face of the chip and external leads of a substrate. The two major pieces of automated equipment, i.e. chip and wire bonders, used to connect the chip and substrate have many variations, but are similar in that the work stations of each apparatus must hold the device under assembly securely against the work surface during the operation.
In the operation of a chip bonder, a substrate is positioned onto a work station where an adhesive, an alloy, or other type of mount material is disposed onto a predetermined location, and a semiconductor chip is positioned atop the adhering material. Typically the work station includes a vacuum chuck having a planar surface of approximately the size and shape of the substrate, and has one or more apertures connected to a vacuum source. Failure to secure the substrate a flat and horizontal position during chip mounting can cause the chip to he misplaced and/or tilted, which subsequently can lead to failures at the wire bonding process.
An automated wire bonder interconnects the input/output (I/O) pads of a semiconductor chip to specified leads on a substrate by fine wires. The substrate must be held tightly against a moveable heater block which serves as a mass forming a fixed and unyielding mandrel so that the semiconductor and the lead to be bonded are firmly clamped. Proper thermosonic wire bonds at the first and second bonding sites are a function of time, force, and ultrasonic power applied through the bonding tool to the wire being bonded. Heating the semiconductor die or substrate is known to reduce the bonding time, and generally improve the bond quality. Failure to securely hold the substrates and chips during wire bonding will lead to poorly formed bonds, broken or lifted wires.
In existing wire bonding apparatus, devices are held by device specific inserts which form the work holder. Typically the inserts are of a metallic composition machined to form a heated work surface with features which conform to the specific device substrate under assembly. A clamp holds the substrate to the work surface. Both the clamp and insert are specific to the dimensions and features of the device under assembly. Vacuum assist may be used to help locate and secure the substrate.
As packaging of semiconductor devices has become more diverse and complex, the devices usually are thinner, and thus the necessity to provide a flat, parallel surface has become more challenging. In older technologies, the substrates were rigid materials, such as ceramics or thick metal lead frames, but in more current devices, substrates often are made of a flexible film, a thin composite or laminate polymer, or a thin lead frame of malleable material which tends to warp and bend under uneven pressure of clamps and large vacuum apertures. Further, substrates may be configured so that multiple devices are assembled in parallel. Edge clamps and work surfaces with vacuum apertures may cause distortion of the flimsy substrates, which in turn can contribute to non-uniform bond line thickness of a paste adhesive, and/or tilted chips during chip mount. Clamps and vacuum apertures in the wire bonder inserts contribute to non uniform heating, as well as broken or poorly formed bonds which is completely unacceptable for reliable, high yield processing.
Not only do the existing bonder work surfaces lack optimum compatibility with current packaging technologies, but also they necessitate a large inventory of device specific parts, and require down time for installation and set-up, which in turn contributes to the cost of assembly, and the device.
In particular, for thin and flexible substrates, it would be advantageous to the industry to have a work station at the chip and wire bond operations which securely holds the device in a planar position, and to have a more universal work station requiring fewer changes than existing wire or die bond equipment.
It is an object of the invention to provide a work station for both wire bond and for die bond apparatus which securely holds the device under assembly in a planar configuration.
It is further an object of the invention to provide a means for securely holding the device under assembly, without use of clamps.
It is an object of the invention that the work station insert will be used for multiple device types within a package family.
It is an object of the current invention to provide a work station insert which is capable of securing thin, flexible substrates.
It is an object of the invention to provide a work station insert which is compatible with different substrate materials and configurations.
It is an object of the invention to provide a work station for assembly of a plurality of devices within the same indexing operation.
It is an object of the invention that the insert will be compatible with existing assembly equipment, as well as newly designed equipment.
It is an object of the invention to provide a thermally conductive insert for wire bonders.
It is an object of the invention that the work surfaces for both wire bonders and chip bonders be cost effective.
It is an object of the invention that the work stations minimize the need for a large inventory of device specific inserts for either chip bonders or wire bonders.
It is an object of the invention to provide a wire bond work station insert which avoids broken wires caused by release of clamps.
It is an object of the invention to provide a work station insert which will contribute to improved device yield, and reliability by providing more uniform support during processing.
The above and other objectives will be met by providing a porous ceramic vacuum chuck work station for a chip bonder wherein the substrate under assembly is securely and uniformly held by vacuum applied through many tiny pores distributed across the entire surface. Similarly, a wire bond work station insert comprised of porous ceramic having high thermal conductivity is provided, which will secure the device under assembly by application of evenly distributed vacuum to the surface. The inserts are applicable to a family of packages, or to a substrate geometry, thereby eliminating the need for numerous device specific clamps, and frequent change-out and set-up with each chip size and configuration. Elimination of clamps from the bonding process obsoletes the previously troublesome broken wire failure as the clamp is moved.
Application of uniform vacuum as a means for securing the device under assembly supports yield and reliability improvement by eliminating warped substrates, and the resulting non-uniform bond line thickness of the adhesive, and by minimizing tilted die. The uniform vacuum hold down during wire bond eliminates xe2x80x9cfloating substratesxe2x80x9d, and the associated failures of lifted stitches and balls.